Fuel assemblies for a nuclear reactor and, in particular, fuel assemblies for a water-cooled nuclear reactor, include, inside a framework, fuel rods which are held together in the form of a bundle in which the rods are mutually parallel.
The fuel rods generally consist of a tube made of an alloy which is a weak absorber of neutrons, such as a zirconium alloy, in which pellets of nuclear fuel are stacked. After the tubes have been filled with the fuel pellets, they are closed at their ends by plugs which are welded to the tube making up the clad of the rod. The fuel-assembly framework itself consists of elements which are mostly made of an alloy which in a weak absorber of neutrons, such as a zirconium alloy. Zirconium and its alloys are passivatable metals, i.e., metals on whose surface a passivation layer, consisting of a very thin oxide, forms naturally, this layer protecting the metal from aggressive external environments.
In the case of fuel rod clads or of other elements of a fuel assembly made of zirconium alloy, this passivation layer establishes in particular the conditions under which ions and electrons are exchanged between the metal of the fuel-assembly elements and the very-high-temperature and high-pressure water for cooling the reactor, while the nuclear reactor is operating. This passivation layer changes during use.
The characteristics of the oxide layer, with regard to the effectiveness of the protection, depend to a large extent on the initial conditions under which this layer was formed at the surface of the zirconium-alloy element.
In general, corrosion inside a nuclear reactor cooled by pressurized water produces, on a zirconium-alloy substrate consisting of sound metal, at least during the first phases of corrosion, a black, shiny, adherent and protective zirconia layer. In contrast, on a metal which does not have the required metallurgical characteristics or which has been contaminated during a manufacturing phase such as welding, corrosion develops by forming a white, non-adherent and non-protective zirconia layer.
It is therefore necessary to verify, by suitable tests, that the zirconium-alloy substrate exhibits satisfactory properties after manufacture of the fuel-assembly element.
In the case of a fuel rod, the ends of which are closed by welded plugs, it is necessary to verify whether the weld zone is completely free of contaminating products. To this end, a 360.degree. three-day corrosion test is performed on the rod in an autoclave, and then the appearance of the passivated surface layer in the region of the plug welds is checked. In the case of a satisfactory contamination-free weld, the passivation oxide layer is uniformly black. The presence of contamination in the region of the weld is manifested by quite extensive white traces or by a greyish coloration.
Such an autoclave corrosion test has drawbacks in the context of industrial-scale manufacture of fuel rods.
The reason for this is that the duration of the corrosion test, taking into account the time for the temperature of the autoclave to rise and fall, is five days, during which the fuel-rod production unit continues production without having the assurance that the welding unit is operating properly. The corrective measures necessary in the case of contamination cannot therefore be taken immediately. This drawback may lead to products being scrapped or significant numbers of products having to be repaired. Furthermore, the units produced may be put into circulation after checking only at the end of the test, which affects the manufacturing flexibility and increases the quantity of semi-finished products in the course of manufacture. Furthermore, the corrosion test is a destructive test which can only be carried out on a small number of specimens per work station. The autoclaves used for the tests are permanently sequestered.
The result of the corrosion test is a purely qualitative all-or-nothing result which does not make it possible to determine the deviation of the production output with respect to the desired standard.
Finally, there is no reliable and quick method making it possible to verify that the welding chamber is correctly purged and that the shielding gas which it contains is correct.